Present Oscillator Problems
All radar systems and virtually all communication systems contain microwave oscillators. The conventional microwave oscillator's noise performance is only moderate, often limiting the entire system's performance. Accurately predicting the performance of the oscillator is often difficult, since the loop phase and loop gain are obscure. Consequently. with the oscillator as the performance limiting factor, the system's performance is usually not known until it has been built and tested. As a result post-tuning of the oscillator and possibly of the rest of the system is usually required.
Regarding the conventional microwave oscillator, two major areas exist which cause the above problems and cause other design and performance problems. These areas of concern are the nonlinear operation of the oscillator and the internal feedback path within the amplifier section of the oscillator.
All oscillators contain amplifiers which are used to build up and sustain oscillation. These amplifiers inherently possess low-frequency noise known as 1/f or flicker noise. This noise is well below the frequency of operation and is not a problem in the linear region of the amplifier. However, in conventional microwave oscillators, the linear region of the amplifier must be exceeded to limit the level of oscillation. Operating the amplifier in the nonlinear region causes an upconversion of the low-frequency noise analagous to the upconversion of audio to a carrier frequency in communication circuits. This noise is now centered around the operating frequency and impedes the performance of the oscillator. Occe the noise is upconverted, it cannot be filtered without attenuating the desired signal as well. To prevent this noise from appearing at the oscillator's output, one must eliminate the nonlinearities present within the oscillator.
Nonlinearities can be eliminated by dynamically controlling the loop gain and thereby restricting the active device within the amplifier to its linear region of operation. However, the conventional oscillator has an internal feedback path that is inaccessable. Therefore loop gain cannot be independently controlled and the conventional oscillator cannot be used to implement a linear oscillator. This poses a major obstacle in improving noise performance. The inaccessible feedback path also causes the difficulty of predicting the oscillator's performance. Since the loop phase and the loop gain information are both obscure, the phase vs. frequency slope and the oscillator's bandpass response cannot be accurately determined. Thus, not only can the oscillator's noise performance not be improved, it cannot even be accurately estimated with the conventional oscillator.
Problems in design accuracy are also encountered due to the nonlinear operation. To account for nonlinearities, one must estimate and use large-signal S-parameters when designing. Since the values of the S-parameters will change as they enter and leave the nonlinear regions of the amplifier, these estimations are average (rather than actual) values. Although large-signal S-parameters do simplify the design process, they also reduce the accuracy of the design and noise performance predictions. As a result, oscillation often occurs at a frequency above or below the expected frequency, thus requiring post-tuning.
Other problems which exist in conventional microwave oscillators include:
the inability to produce a wide-band oscillator due to the inability to provide a amplifier which is unstable over a wide frequency range, PA1 poor FM noise performance at higher frequencies due to a drop in Q and thus lower phase vs. frequency slopes at the higher frequencies, and PA1 the inability to produce monolithic low-noise oscillators due to the excessive flicker noise in GaAs MESFETs. PA1 the ability to accurately predict the phase vs. frequency slope of the oscillator and thereby achieving the ability to determine the amount of FM noise which will occur as a result of inherent phase noise, PA1 the ability to increase the phase vs. frequency slope and further reduce FM noise by simply adding integer wavelengths of delay within the feedback loop, PA1 the ability to use conventional amplifiers and amplifier techniques since the amplifier operates in the stable region, PA1 the ability to verify at a glance that oscillation is not possible at frequencies other than the intended frequency of operation, PA1 the ability to construct a multiple frequency oscillator from a single amplifier by multiplexing filters of differing center frequencies, PA1 the ability to create a voltage-controlled oscillator with either a voltaged-controlled filter or a voltage-controlled phase shifter since a change in either the loop phase or the loop gain will change the frequency of operation, PA1 the ability to create an amplitude-modulated carrier from a single active device and with no additional circuitry, by modulating the gain of the amplifier. PA1 the ability to create a frequency-modulated carrier with the addition of only a phase shifter in the feedback loop, and PA1 the ability to control the output power of the oscillator by adjusting the loop gain of the oscillator. PA1 no estimation of the large-signal S-parameters is necessary, allowing the oscillator to be accurately designed with small signal s-parameters, PA1 low-frequency flicker noise is virtually eliminated by linear operation, PA1 harmonics created by nonlinear amplification are eliminated, PA1 low-noise monolithic oscillators may be produced with GaAs MESFETs in spite of their poor flicker noise performance, PA1 prediction and control of the exact final dc operating point is inherent and is often desirable for noise and power output criteria, and PA1 the exact power output may be directly predicted since the precise bias point is known.